Evolutionary ecology of brains Flashcards
Why do we look at brain size?
Easily measured
Can be predicted by fossil record, although some taxa have a poor fossil record e.g. primates (best to model using Bayesian analysis)
Provides an estimate of some other neural parameter/s responsible for intelligence
Brain and body and constraints
Brain size scales with body size - Jerison thought it was constrained by SA:V ratio of body, Martin thought basal metabolic rate
Brain growth and body growth have different ontogenic trajectories - e.g. in mammals, brain growth ceases long before body growth.
Brain/body allometry is specific to each taxon
Balaban et al 1988 - Inter-specific transplantation experiments in birds showed body size does not dictate brain size.
Metabolic cost of brain in vertebrates is 2-10% of total. 20% in humans, despite being 2% of body mass
BMR relative to body mass increases with relative brain size
Evidence selection can act on total brain size - experimental
We can artificially select for big or small brains in mice and guppies
Kotrschal et al - developed guppy lines that differed in relative brain size by 9%. Found large-brains improved numerical learning ability in females, and spatial learning ability in males. Later found large brains conferred a more proactive personality. But small-brained animals had lower gut mass and fewer offspring.
Evidence selection can act on total brain size - phylogenetic analyses
Looking at primate phylogeny, a model biased towards directional selection fit brain size evolution best, while a model of random direction of change fit body mass evolution best.
Montgomery et al 2010 - systematically analysed primate ancestral phylogeny. whilst brain size decreases along some phylogenetic lines, overall absolute and relative brain size increases, whilst body size does not. The human lineage is interesting only in absolute brain mass increase.
Evidence selection can act on total brain size - molecular
Large-brains in guppies was associated with increased expression of Ang-1, which controls neurogenetic activity of neural progenitor cells
Human specific changes in FoxP2 have increased synaptic plasticity in circuits surrounding language and speech
In microcephaly, asymmetric division of neural progenitors begins too early. Caused by mutations in ASPM (which interacts with CITK to organise and orient the astral microtubules). ASPM coevolved with brain mass but not body mass
(remember selection–>genes–>brain size
Evidence that overall brain size relates to cognition
Innovation = spontaneous production of a new behaviour in response to environmental stimulus. Innovation correlates with relative brain size in primates and birds
Absolute brain mass correlated with ‘global cognition’ in primates, as measured by 9 tests
Absolute brain mass correlates with success in self-control tests
Relative brain size and absolute brain mass correlated with problem-solving tests
Kotrschal et al’s bigger brained guppies were better at numerical (female) and spatial (male) tests
Problems with evidence linking overall brain size to cognition
Behavioural tests assume trial+error or associative learning as equivalent to understanding - Logan 2016 found no correlation in individual performance in innovation and reversal learning tasks.
Global cognition appears to explain less than 40% of variation. Correlation between g and brain size in humans is very weak (r=0.2-0.3)
Why should animals pay attention to humans trying to train them? Why do we judge intelligence by human standards/tests? Must take into account ecological differences between taxa when measuring ‘intelligence’
Brain size may not matter (think neuronal density, cellular characteristics, genetics)
For concerted evolution
Concerted brain hypothesis - allometry between regions is due to order of neurogenesis, e.g. neocortex develops later and hence is disproportionately bigger as overall brain size increases. Developmental conservatism constrains brain structure.
Brain scaling is conserved across taxa, and order of neurogenesis is conserved between mouse and monkey (this suggests strong developmental constraints, and component size determined by brain size. Selection acts through brain size)
For mosaic evolution
There are grade shifts in relative neocortex size between taxa (grade shifts mean altered y-intercept, which in a brain size-body size graph will be log(allometric constant), constant thought to be the encephalisation quotient (Jerison). When effects of grade shifts are taken into account, neocortex scales nearly isometrically
Partial correlations among individual components vary between groups, matching functional connectivity
Functional coevolution pervades biological levels, and is apparent in volume of components, volume of subcomponents, and cellular composition (neuron density varies across brain regions, so increasing mass of different regions will increase overall brain size to different extents)
Interspecific variation in component size (e.g. primates have higher neuron densities in the cerebellum and neocortex than other mammals) is more strongly correlated with ecology than overall brain size
Patterns of covariance between components can themselves be selected on.
(suggests strong functional constraints, and component size determined independently of brain size. Selection acts on individual brain components)
Evidence against size mattering
When human FoxP2 was put in mice, they had improved performance in learning tasks and altered development of basal ganglia, but NOT bigger in any region of the brain.
Han et al 2013 - Grafting human glial progenitor cells into neonatal mice led to improved LTP and improved learning. so why should we focus on neuron density/number?!
Can’t ignore cellular composition/properties - elasmobranchs have much bigger brains than teleosts but much lower energy consumption because less Na/K pump.
Neuron number and volumetric proportions are uncorrelated
It is circular to argue that size, or executive brain ratio, or neocortex size is a good measure of cognition simply because that measure supports your hypothesis.
Expensive brain/tissue hypothesis
Expensive brain hypothesis - across primates, relative gut size is inversely correlated with relative brain size
Kotrschalz’s large-brained guppies had smaller guts
Expensive tissue hypothesis - the actual tradeoff is with adipose deposits
When fat is included separately in the analysis, the gut increases in size with the brain.
Investment in fat and big brains may be redundant safeguards against unpredictable food supply
Constraints lifted by big brains
Mammals with bigger brains live longer - perhaps longer memory needed and behavioural flexibility because they’ll experience more changes? Does brain size limit lifespan, or does lifespan limit brain size? Latter possible, but in mammals brain stops growing in adolescence.
Requirement for large fat stores
…requirement for many offspring?
Adjustments required to support big brains
Longer gestation and increased maternal energy turnover to support neurogenesis
Increased lactation duration to support synaptogenesis, axon and glial growth
Birds with large relative brain size have longer incubation periods, later fledgling ages, longer post-fledling care. More altricial development may require more biparental care, which is supported by increased pair bonding.
–Gonzales-Voyer et al 2009 - female but not male brain size in cichlids was increased with uniparental care, i.e. the cognitive load was on the mother. –
Migratory distance decreases as relative brain size increases across bird species
Increased relative brain size in carnivores is associated with diet quality (e.g. insectivores have smaller brains than carnivores)
Relative brain size in primates correlates with frugivory - fruits are more calorific and easier to digest. BUT frugivores also have wider home ranges, is the correlation just because bigger brain + spatial memory allows frugivory? No, because hippocampus size does not correlate with home range, and brain size does not correlate with home range size when controlling for frugivory.
Increased food processing - apes preferentially eat cooked meat when given the choice, and mice grow better if you feed them cooked food. But this one’s hard to tell if it’s allowing or requiring a big brain…
Specific examples supporting mosaic evolution
Cave fish have smaller optic tectum than surface-dwelling fish, but not smaller brain overall.
In sticklebacks, heritability of relative size of brain regions was low, and genetic correlation was low. having low genetic covariance reduces likelihood that evolution of one region will be constrained by change in a correlated region reducing fitness.
Primates had high heritability in brain structures only after correcting for total brain size (i.e. so variation is not by some overall total brain size gene)
Potential genetic mechanisms of mosaic evolution
Genes might:
shift boundaries in progenitor pool prior to neurogenesis (to alter relative sizes but not overall size. Demonstrated in African cyclids; morphogen patterning along rostro-caudal axis increases telencephalon)
region-specific increased duration of neurogenesis (telencephalic neurogenesis is delayed in passerine birds, which increases number of cells destined for telencephalon)
region specific increased rate of neurogenesis (Period of accelerated cell cycling occurs in galliform birds. Differences in genes linked to human brain expansion have been shown to alter cell cycling)
Brain expansion and sensory adaptation
Brain regions responsible for the same sensory modality co-evolve, those for different modalities do not.
Sensory specialisation in brain structure (as seen in cave fish) is due to ecology, not a trade-off - frugivory in nocturnal primates expands olfactory cortex, in diural primates expands visual cortex, but in bats expands both. Bats show it’s not a biological constraint, there’s just not a selection pressure to have both in the primates.
Degree of binocularity in primates correlates with total brain size, and with relative number of parvocellular neurons in LGN. Relative size of cortical and subcortical visual structures correlates with overall neocortex size and relative brain size. So maybe visual evolution accounts for expansion?
In humans, V1 is reduced relative to rest of neocortex, but parietal areas in dorsal stream (magnocellular) expanded compared with macaques. So primates have evolved for detail, we’ve evolved away from this towards movement?
Relative visual cortex size in primates correlated with relative facial nucleus size, which also correlated with grouo size. Links sensory theory to social theory?
Social brain hypothesis - evidence linking big brains to social behaviour
Dunbar and Schultz, 2007 - relative neocortex size correlates with group size in primates, esp with female cohort size
relative neocortex size is higher in pair-bonded bird species
Maybe the primate effect is because friendships are basically platonic pair-bondings?
Individual differences in group size correlate with volume differences in frontal and temporal regions in humans and macaques
Primates with larger brains have higher rates of social learning
Primates with a larger neocortex show more tactical deception
Kotrschalz et al - big brained guppies exhibited more proactive behaviour
Machiavellian Intelligence Hypothesis
Groups are intrinsically competitive and exploitative.
But e.g. herd of cows aren’t
If groups are so exploitative, why stay in them? SO MIH is effect, not cause, of social group size
Social brain hypothesis - why live in groups?
Living in big groups can’t be dismissed as a byproduct of big brains because it is so costly to live in a big group - competition, stress, time sink.
female baboon longevit and fecundity correlate with number of social partners, so social groups improve fitness, so the bigger brain required is worth the cost.
Social brain hypothesis - mechanism behind it
A problem is solved socially
A big brain is required to achieve the requisite sociality
It’s the complexity of these relationships that is important. Sociality is ‘building and maintaining complex social relationships’. Simple group size correlation is only found in primates… perhaps because their interactions are by default complex?
Brain expansion and foraging style
Relative brain size in primates correlated with frugivory, and not with home range when controlling for frugivory.
Frugivory correlates with neocortex size when controlling for home range size.
No association between hippocampus and home range size
Controlling for group size erased the frugivory-brain size correlation. So perhaps big groups need big brains which need frugivory
Extractive foraging (using tools) correlated with executive brain ratio in primates. BUT innovative foraging is either abundant in a species or not there at all, so cannot account for continuous variation in brain size
Neocortex and cerebellum coevolve, and both correlate with relative brain size. One exception is the cerebellar expansion seen in primates and other extractive foragers. Both are likely to contribute to both social and physical intelligence.
Summary of ecological pressures on brain size
Activity patterns (diurnal vs nocturnal) Diet Group size Method of foraging (innovative/extractive) Stereoscopic convergence
It’s likely all of these selective pressures cause complex patterns of coevolution in independent brain structures, patterns which vary between species. Remember selection acts on the behaviour/system, not on any neuroanatomical parameter.
Genetic explanations for primate brain evolution
I.e. we have a gene mutation that allows big brains.
For - FOXP2, Ang-1
Against - low genetic correlation between different brain regions
Developmental explanations for primate brain evolution
More parental investment allows bigger brains
For - birds that exhibit biparental care have bigger brains. Humans have longer post-natal dependence on parents, longer gestation, longer duration of lactation. For females, more parental investment means a bigger brain (i.e. bigger female brains in uniparental cichlids).
Against - In humans, the maternal brain shrinks by up to 7% during gestation (Oatridge et al 2002). Extensive biparental care is seen in a wide variety of small-brained animals. Brain size is negatively correlated with fecundity (guppies). Ebneter et al 2016 found that increased prenatal maternal investment in the Japanese quail negatively correlated with cerebellum size, so parental investment could limit brain size, or vice versa. But others found that birds with larger brains have longer incubation time, older fledgling age, so maybe the quail thing was because they were artificially selected?
